I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for data transmission using time gated frequency division duplexing (FDD).
II. Description of the Related Art
A communication system is typically required to support multiple users sharing the same resources. One such communication system is a code division multiple access (CDMA) system which conforms to the "TIA/EIA/IS-95A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", hereinafter referred to as the IS-95A standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The CDMA system is a spread spectrum communication system. The benefits of spread spectrum communication are well known in the art and can be appreciated by reference to the above cited references. The CDMA system is designed to work within the pre-existing non-contiguous frequency allocation in the cellular band. By design, a CDMA system which conforms to IS-95A standard is allotted a 1.2288 MHz bandwidth to fully utilize the cellular band.
The CDMA system is capable of transmitting data traffic and voice data over the forward and reverse links. A method for transmitting data traffic in code channel frames of fixed size, wherein the data source provides data at a variable rate, is described in detail in U.S. Pat. No. 5,504,773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95A standard, the data traffic or voice data is partitioned into code channel frames which are 20 msec wide.
In the CDMA system, users communicate with one another through remote stations which, in turn, communicate with each other through one or more base stations. In this specification, base station refers to the hardware with which the remote stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one remote station communicates to a second user on a second remote station, or a standard telephone, by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station or a public switched telephone network (PSTN). If the second user is on a remote station, the data is transmitted on the forward link of the same base station, or a second base station, to the second remote station. Otherwise, the data is routed through the PSTN to the second user on the standard phone system. The forward link refers to transmission from the base station to a remote station and the reverse link refers to transmission from the remote station to a base station. In IS-95A systems, the forward link and the reverse link are allocated separate frequencies and are independent of one another.
The remote station communicates with at least one base station during a communication. CDMA remote stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a remote station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are typically serviced by the same base station. The process of softer handoff is described in detail in U.S. patent application Ser. No. 08/405,611, entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION", filed Mar. 13, 1995, assigned to the assignee of the present invention and incorporated by reference herein.
In softer handoff, the base station transmits identical forward link signals over multiple sectors of the same cell. Because the base stations serving the sectors are physically located at the same location, one common set of hardware can be designed to support softer handoff at the base station.
The signal transmitted by the base station or the remote station can reach the destination device through one or more propagation paths.
Furthermore, in soft handoff the forward link signals are transmitted from multiple base stations. The demodulation of multiple code channels during soft handoff and of multipath signals is described in detail in U.S. Pat. No. 5,109,390, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,490,165, entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF
RECEIVING MULTIPLE SIGNALS", both assigned to the assignee of the present invention and incorporated by reference herein.
The diversity receiver disclosed in U.S. Pat. No. 5,109,390 is also referred to as a rake receiver. The rake receiver contains multiple fingers with each finger comprising one correlator. At the remote station, each correlator has the capability to despread the received baseband signal with a unique set of short PN codes and a unique Walsh code which are assigned to that particular correlator by the remote station. Not all correlators are in use at all times. In fact, only the outputs from the correlators which have been assigned by the remote station are combined by a combiner to provide a more reliable estimate of the transmitted signal. The combined signal is then decoded by a subsequent decoder.
At the remote station, the same rake receiver can be used to demodulate multiple signals from soft handoff and softer handoff. The only difference to the remote station is that in softer handoff, the base stations are configured to send identical reverse link power control bit values to the remote station since the base stations are physically situated at the same location. The power control bit commands the remote station to adjust its transmit power up or down to maintain the desired level of performance while minimizing the level of interference to other users. In soft handoff, the base stations do not send the same power control bit values because they are typically situated at different locations. This difference can be accounted for in the design of the rake receiver. In the exemplary embodiment, combiner 180 coherently combines the scalar outputs which correspond to the data
Application specific integrated circuits (ASICs) have been designed to support soft and softer handoff for the base station and the remote station. An example of a remote station ASIC which supports soft handoff is the Qualcomm Q5257 Mobile Station Modem (MSM). A base station ASIC capable of supporting softer handoff is the Qualcomm Q5160 Cell Site Modem (CSM). The design and capabilities of these ASICs are disclosed in U.S. patent application Ser. No. 08/492,592 entitled "MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM", filed Jun. 20, 1995, and U.S. patent application Ser. No. 08/572,632 entitled "CELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM", filed Jan. 13, 1995, both assigned to the assignee of the present invention and incorporated herein by reference.
In the exemplary CDMA communication system, data transmission occurs concurrently on the forward link and the reverse link. This is possible since the forward link and the reverse link are allocated separate transmission frequencies. Concurrent transmission and reception can require additional hardware which can be simplified or eliminated through a time gated transmission scheme. Furthermore, concurrent transmission and reception may not be possible if separate frequencies do not exist.